1. Field of the Invention
The present invention relates to a capillary used in a wire bonding apparatus.
2. Prior Art
A typical wire bonding method in the process of manufacturing, for instance, semiconductor devices, includes the steps shown in FIGS. 4(a) through 4(g).
First, as shown in FIG. 4(a), a ball 3a is formed by a spark discharge created by an electric torch 5 on a wire 3 which extends from the lower end of a capillary 4, and then the electric torch 5 is moved in the direction shown by the arrow. Next, as shown in FIG. 4(b), the capillary is moved to a point above the first bonding point 1a; and then as shown in FIG. 4(c), the capillary 4 is lowered, the ball 3a on the tip end of the wire 3 is pressed against the first bonding point 1a, and an ultrasonic vibration is applied to the capillary 4 by a horn which has thereon the capillary 4 so as to bond the ball 3a to the first bonding point.
Afterward, as shown in FIG. 4(d), the capillary 4 is raised and as shown in FIG. 4(e) moved in the looping direction A so as to be positioned at a point above the second bonding point 2a. Next, as shown in FIG. 4(f), the capillary 4 is lowered, the wire 3 is pressed against the second bonding point 2a, and an ultrasonic vibration is applied to the capillary 4 by the horn so as to bond the wire 3 to the second bonding point 2a. Then, after the capillary 4 is raised to a given position, a damper 6 is closed, the capillary 4 and damper 6 are raised together, and the wire 3 is cut as shown in FIG. 4(g).
The connection of one wire is thus completed.
Japanese Patent Application Laid-Open (Kokai) No. S57-87143 and Japanese Patent Application Publication (Kokoku) No. H1-26531 disclose examples of the wire bonding methods described above.
The capillary 4 described above is taught by, for instance, Japanese Utility Model Application Publication (Kokoku) No. H1-42349 and Japanese Patent Application Publication (Kokoku) No. H3-780. The details of the structure of a capillary of this type is described in FIG. 3.
In FIG. 3, two chamfers, i.e., lower and upper chamfers 11 and 12, are formed near the tip end of a wire threading hole 10 of the capillary 4 through which a bonding wire passes. In FIG. 3, the reference numeral 13 referrs to a portion where the lower chamfer 11 is formed continuously to the upper chamfer 12; and in addition, HD indicates the diameter of the wire threading hole 10, T indicates the tip diameter of the capillary 4, CD.sub.1 indicates the diameter of the lower chamfer 11, .theta..sub.1 indicates the chamfer angle of the lower chamfer 11, .theta..sub.2 indicates the chamfer angle of the upper chamfer 12, .alpha. indicates the face angle, and OR indicates the outer radius. When d is the diameter of a bonding wire 3, HD is d+(8 to 12) .mu.m (microns); and .theta..sub.1 is 80 to 100 degrees, and .theta..sub.2 is 20 to 40 degrees.
In the prior art capillary 4 described above that has two chamfers, the lower and upper chamfers 11 and 12, when the ball 3a is bonded to the first bonding point 1a shown in FIG. 4, the excess portion of the ball flows toward the wire threading hole 10 through the upper chamfer 12 and wells up inside the capillary 4. As a result, the diameter of the press-bonded ball is reduced. However, pads on pellets are installed at finer pitches due to today's demand of higher integration of semiconductor IC chips; and as a result, both the diameter of the wire 3 and the size of the ball 3a are also required to be reduced. The prior art described above can be used in cases where the diameter d of the wire 3 is in the range of 20 to 80 .mu.m; however, if the diameter d of the wire 3 is as small as in the range of 10 to 30 .mu.m, various problems arise.
When the chamfer angle .theta..sub.2 of the upper chamfer 12 is large, i. e., if it is in the range of 20 to 40 degrees, then it is likely that the amount of ball material which wells up into the capillary 4 (i.e. the amount of ball material that enters the inside of the capillary 4) during press-bonding of the ball increases. As a result, the amount of ball material that is crushed and press-bonded by the lower chamfer decreases, and the pressing force during press-bonding of the ball to the first bonding point 1a by the capillary 4 decreases, resulting in a drop in the bonding force. Furthermore, since material of the ball 3a enters the wire threading hole 10 of the capillary 4, friction is caused when the capillary is raised; and this results in a force which causes peeling of the press-bonded ball bonded to the first bonding point and further a force which reduces the press-bonding force.